1. Field of the Invention
The present invention relates to a plasma ignition device used for an ignition-difficulty combustion engine.
2. Description of Related Art
In recent years, fuel lean and high-supercharge combustion engines for vehicles have been steadily progressed in order to reduce environmental load substance contained in exhaust gas and to further improve fuel efficiency. Generally, since fuel lean combustion engines and high-supercharge combustion engines are relatively difficult to ignite, there is a strong demand for an ignition device which improves the ignition performance of such engines.
To address this demand, there are proposed various plasma ignition devices as next-generation ignition devices configured to inject a gas in a high-temperature and high-pressure plasma state into a combustion chamber to assure stable ignition operation in the fuel lean combustion engines or high-supercharge combustion engines having difficulty in igniting by use of a conventional spark plug. For example, refer to Published Japanese Translation No. 2000-511263 of PCT International Publication, or Japanese Patent Application Laid-open No. 2006-294257, or Japanese Patent Application Laid-open No. 2008-177142.
Such plasma ignition devices have a structure in which a high voltage is applied between a pair of opposed electrodes of an ignition plug defining a discharge space therebetween to cause a breakdown discharge therein by breaking down the insulation in this discharge space, the breakdown discharge triggering supply of a large current into the discharge space so that the gas in the vicinity of a discharge path formed by the breakdown discharge is brought to the plasma state, followed by the combustion chamber being injected with fuel to generate a flame core of large volume in the combustion chamber.
For example, the conventional plasma ignition device 1z shown in FIG. 17 includes an ignition plug 10z, a brake discharge circuit 30z to apply a high voltage to the ignition plug 10z, a plasma discharge circuit 40z to supply a large current to the ignition plug 10z, and an ECU (Electronic Control Unit) 60z to generate an ignition signal to control the breakdown discharge circuit 30z and the plasma discharge circuit 40z in accordance with the running state of a combustion engine.
The breakdown discharge circuit 30z includes a step-up coil 31z to step up a voltage of a power supply 20z, a step-up coil drive circuit including a switching element 32z to open and close the step-up coil 31z, a rectifying element 33z to rectify a current flowing from the step-up coil 31z to the ignition plug 10z, and a noise-absorbing resistor 34z to absorb high-frequency noise generated when ignition is carried out.
The plasma discharge circuit 40z includes a capacitor Cz to accumulate electrical energy supplied from the power supply 20z, a charge resistor 411z to limit a current flowing from the power supply 20z to the capacitor Cz to an appropriate value, and a large-capacity rectifying element 430z to rectify a plasma discharge current discharged from the capacitor Cz. The voltages supplied to the breakdown discharge circuit 30z and the plasma discharge circuit 40z are adjusted to appropriate values by a voltage-regulating circuit 22z such as a DC/DC converter.
When an ignition switch 21z is closed, the low voltage supplied from the power supply 20z is applied to the primary side of the step-up coil 31. When a primary current Iprz is interrupted by the switching element 32z in accordance with the ignition signal IGtz outputted from the ECU60z, a high secondary voltage Vscz (10-30 kV, for example) is generated at the secondary side of the step-up coil 31z in the direction to prevent change of the magnetic flux in the step-up coil 31z. 
When the secondary voltage Vscz exceeds the withstand voltage in the discharge space defined in the ignition plug 10z, there occurs a breakdown discharge, breaking down the insulation in the discharge space. Subsequently, triggered by this breakdown discharge, the electrical energy accumulated in the capacitor Cz is discharged into the discharge space as a large current, as a result of which the gas in the discharge space is injected into the combustion chamber in a high-temperature and high-pressure plasma state. Since the gas in the plasma state, which is large in volume, generates a frame core of high energy and high combustion speed, the above plasma ignition device is expected to stably ignite air-fuel mixture in the combustion chamber of an ignition-difficulty combustion engine.
Further, the above plasma ignition device is expected to perform stable ignition operation in not only an internal combustion engine of a vehicle, but also a cogeneration system for generating power using gas fuel.
However, the above conventional plasma ignition device 1z has a problem in that if the charge voltage Vcz of the capacitor Cz is set below a relatively low voltage, below 400 V, for example, discharge from the capacitor Cz does not start unless the discharge voltage applied from the breakdown discharge circuit 30z to the ignition plug 10z after insulation breakdown decreases below 400 V.
Since the discharge current flowing from the breakdown discharge circuit 30z to the ignition plug 10 is as small as below 100 mA, the interelectrode voltage of the ignition plug 10 having a negative resistance does not become below 400 V stably.
Accordingly, as shown in FIG. 18A, since the discharge voltage after insulation breakdown may become higher than 400 V before discharge from the capacitor Cz is started due to pressure variation in the discharge space, there may occur the so-called plasma-dropout phenomenon in which no discharge from the capacitor Cz occurs. If the plasma-dropout phenomenon occurs, since no large current is supplied from the capacitor Cz into the discharge space, there is a possibility that the gas in the discharge space is not injected into the combustion chamber causing misfire, preventing the in-cylinder pressure CYL from increasing.
If the charge voltage Cvz of the capacitor Cz is set to a high voltage, above 800 V, for example, since the interelectrode voltage of the ignition plug after insulation breakdown is stably below 800 V because of a discharge current from the breakdown discharge circuit 30z, a discharge from the capacitor Cz is started without fail, causing the gas in the plasma state within the discharge space to be injected into the discharge space.
However, in this case, since the ignition plug 10z is in a state of always being applied with the relatively high voltage, if the in-cylinder pressure decreases due to opening and closing of the discharge valve and the inlet valve of the engine, or descent of the piston of the engine as a result of which the insulation resistance in the discharge space decreases, there may occur the so-called false discharge irrespective of the ignition signal IGtz as shown in FIG. 18B.
Since such a false discharge causes a large current to flow from the capacitor Cz, the electrodes of the ignition plug may be worn out rapidly. Further, such a false discharge causes energy waste. Further, when a false discharge occurs during an air inlet period, the engine may be broken down due to early ignition.
Incidentally, if the charge voltage of the capacitor Cz is set between 400 V and 800 V, either the plasma plasma-dropout phenomenon or a false discharge may occur indeterminately depending on the running state of the engine.
In addition, to set the charge voltage of the capacitor Cz above 800 V to prevent the plasma-dropout phenomenon, it is necessary to provide insulation for ensuring safety around high voltage sections. For example, a high-withstand voltage cable and high-withstand voltage connectors have to be used for connection between the DC/DC converter 22z and the plasma discharge circuit 40z, and a high-withstand voltage capacitor of large capacitance has to be used as the capacitor Cz. This increases the size of the plasma ignition device, making it difficult to be mounted on a vehicle.
Although cogeneration systems generally have a large installation space which allows installation of a plasma ignition device large in size including a large capacitor to accumulate plasma energy, if the capacitor is applied with a high voltage for a long time period, a false discharge may occur due to change of the insulation withstand voltage of the discharge space caused by pressure variation in the combustion chamber.